This invention relates to a method of forming electron gun electrodes which have the same performance and are constructed integrally as in the three electron guns of a color picture tube.
As shown in FIG. 1, a conventional in-line type electron gun structure utilized in a color picture tube comprises a flat plate shaped cathode holder 1 and three parallel cathode electrodes 2 heated by cathode heaters 3 contained therein for emitting electron beams. In front of the cathode electrodes 2, there are serially disposed a first grid electrode 4 for controlling the electron beams, a second grid electrode 5 for accelerating the electron beams, third and fourth grid electrodes 6 and 7 constituting an electron lens, which are supported by a bead glass rod 8. The electron beams passing through the electron lens impinge upon phosphor picture elements on the inner surface of the panel of the color picture tube, not shown.
The third and fourth grid electrodes are also called main lens electrodes and provided with a pair of three cylindrical projections 6a, 6b, 6c and 7a, 7b, 7c, each pair being formed in opposing cylindrical grid electrodes 6 and 7 and corresponding cylindrical projections being in axial alignment. The length L and the inner diameter D of each cylindrical projection should have a ratio larger than above 0.5 and the degree of true circle of the inner bore of the cylindrical projection should have an accuracy of less than 40 microns.
It has already been proposed to manufacture the main lens electrodes by integrally forming three lenses each having cylindrical projections having a ratio L/D larger than 0.5 as disclosed in Japanese Preliminary Publication of Pat. No. 66840/1976 dated May 20, 1970. According to this method, at first, perforations 12 having a predetermined inner diameter d1 are formed through a blank plate 10 as shown in FIG. 2A. Then as shown in FIG. 2B cylindrical projections 14 are formed by squeezing. At this time, the inner diameter d1 of the openings 12 is increased to d2 due to elongation of the blank. Then, openings 16 having larger inner diameter d3 are formed as shown in FIG. 2C. Then as shown in FIG. 2D, top ends of the projections are removed by boring or burring to obtain top opened cylindrical projections. Under this state, stress caused by the boring remains in the cylindrical projections 18 which results in an elastic deformation tending to decrease the inner diameter of the top openings 19, so that it is impossible to obtain a degree of true circle of the inner diameter of less than 40 microns that is required for the main lens electrodes. For this reason, the inner wall 20 of each cylindrical projection 18 is subjected to a strong squeezing operation to enlarge the inner diameter to D. As a consequence, it is impossible to form lenses 22a, 22b and 22c having cylindrical projections 21a, 21b and 21c and having a ratio L/D of larger than 0.5.
With this method, however, since strong squeezing force is applied to the cylindrical projections to plastically deform them, when one of the cylindrical projections 21a, 21b and 21c shown in FIG. 2E is depicted in an enlarged scale it can be shown in FIG. 3. Thus, as a result of strong squeezing force, the inner surface of the cylindrical projection is elongated extremely to form a sharp edge 23 at the upper end of the opening 18. This sharp edge 23 is not uniform throughout the entire periphery of the opening 19 and often takes the form of irregular wavy form. This decreases the degree of true circle thus affecting the focussing characteristic of the color picture tube. Where jigs are inserted into the openings 18 for assembling the main lens electrodes, the jigs may contact with the sharp edges 23 thus degrading the true circle. For this reason, the yield of the lens electrodes having a high accuracy would be decreased. When the sharp edges 23 are tumbled to eliminate them, the edges would project into openings 18 so that it is necessary to remove the projected edges by hand work.